Patent document 1 described below discloses an invention relating to a method for producing a sealed letter by folding in three. An envelope sheet which is substantially similar to a sealed letter form used in this invention is shown in FIGS. 31 and 32. As shown in FIG. 31, this envelope sheet 300 includes three paper pieces: an upper paper piece 301, a middle paper piece 302, and a lower paper piece 303, connected by perforations. Name and address are written on the front side of the upper paper piece 301, contents are written on the front sides the middle paper piece 302 and lower paper piece 303. On the front side of the middle paper piece 302 and lower paper piece 303, a pressure-sensitive adhesive 305 is formed in an approximate square U-shape and in a band-shaped pattern along the external forms of two paper piece other than perforations. No pressure-sensitive adhesive is present on the front side of the upper paper piece 301 which will finally be the other surface. Moreover, nothing is written on the back sides of the upper paper piece 301, the middle paper piece 302 and lower paper piece 303, or a printing pattern is formed to prevent the content from being seen through. On the back sides of the upper paper piece 301 and middle paper piece 302, the pressure-sensitive adhesive 305 is formed in a band-shaped pattern along the outer edge other than perforations and part of the sides. No pressure-sensitive adhesive is present on the back side of the lower paper piece 303 which will be the outer back side finally.
After name, address, contents and other information are printed, in the case where the envelope sheet 300 is formed into the shape of the envelope, the front side of the middle paper piece 302 and the front side of the lower paper side 303 are placed together as shown in FIG. 19 and the pressure-sensitive adhesives 305 on both paper pieces are brought into contact with eachother. Furthermore, as shown in FIG. 32, the back side of the upper paper side 301 and the back side of the middle paper side 302 are placed together to bring the pressure-sensitive adhesives 305 on both paper pieces into contact with each other. The envelope sheet 300 is in the form of a folded-in-three envelope in this state, but the pressure-sensitive adhesives 305 are not adhered yet.
Thereafter, as shown in FIG. 4 of the above patent document 1, the envelope is nipped by a pair of rollers whose size in the axial direction is greater than their length in the width direction of the envelope, and is transferred while applying a high pressure on the entire surface of the envelope. In FIG. 4 of the above patent document 1, a pressure is applied to the entire surface of the envelope by two pairs of rollers which are longer than the width of the envelope. Accordingly, the pressure-sensitive adhesives which are in contact with each other between the paper pieces facing each other develops tackiness, and a sealed envelope, that is, a sealed letter is produced.
In the technique described above, a sealed letter is produced by folding the envelope sheet, a pressure-sensitive adhesive which requires pressure has been used for the adhesion of the paper pieces of the envelope sheet. In addition, other adhesion sections include a method of applying an adhesive such as hot melts immediately before folding the envelope sheet.    Patent document 1: Japanese Patent Publication No. 2521498